(1) Field of the Invention
This invention relates to a substrate usable for an acoustic surface wave device which includes a base material made of C-faced sapphire single crystal and an AlN film formed on the base material by a metal organic chemical vapor deposition (MOCVD) method using trimethylaluminum and ammonia as raw material gases. This invention also relates to a method for fabricating the same substrate, and an acoustic surface wave device having the same substrate.
(2) Related Art Statement
An AlN film can be made on a base material of C-faced sapphire single crystal by an MOCVD method under a lower pressure using trimethylaluminum and ammonia as raw material gases, as disclosed in xe2x80x9cJournal of Crystal Growthxe2x80x9d, 115(1991), pp.643-647. Such an AlN film serves as a buffer layer, and the thus obtained substrate made of the base material and the AlN film may be preferably usable for an acoustic surface wave device because the AlN film has a larger band width of 6.2 eV, a higher heat conductivity and a thermal expansion coefficient similar to the one of Si or GaAs.
In the above document, concretely, the AlN film is made as follows. First of all, a C-faced sapphire substrate is set on a susceptor installed in a reactor, and heated to 1200xc2x0 C. Then, a trimethylaluminum and an ammonia are introduced as raw material gases with a carrier gas composed of hydrogen gas into the reactor and supplied onto the substrate, to form the AlN film oriented in the C-axis through the reaction between the raw material gases. The ratio (ammonia/trimethylaluminum) is set to 2xc3x97104.
As mentioned above, the substrate composed of the AlN film and the base material of C-faced sapphire single crystal exhibits excellent performance as a substrate of an acoustic surface wave device, but has relatively poor surface flatness. Therefore, the substrate cannot be practically usable for the acoustic surface wave device as is. For practical use, the substrate is required to be polished. As a result, the fabricating process of the substrate is made complicated, and the fabricating cost of the substrate is increased.
Moreover, since the ratio (ammonia/trimethylaluminum) is set to 2xc3x97104 it is difficult to control the fabricating condition, so that the crystal quality of the AlN film is deteriorated. For example, the full width at half maximum (FWHM) in X-ray rocking curve of the AlN film is more than 100 arcsec, and thus, the substrate having such a low quality AlN film is not practically usable for an acoustic surface wave device.
It is an object of the present invention to work out the above conventional problems, and thus, to provide a substrate usable for an acoustic surface wave device which has excellent surface flatness, in addition to excellent crystal quality. It is another object of the present invention to provide a method for fabricating the same substrate stably without polishing and an acoustic surface wave device having the same substrate.
In order to achieve the above object, this invention relates to a substrate usable for an acoustic surface wave device, including a base material made of C-faced sapphire single crystal, and an AlN film having crystallinity of 90 arcsec or below in FWHM of X-ray rocking curve and a surface flatness of 20 xc3x85 or below which is formed by an MOCVD method using trimethylaluminum and ammonia as raw material gases.
This invention also relates to a method for fabricating a substrate usable for an acoustic surface wave device, including the steps of preparing a base material made of C-faced sapphire single crystal, and forming, on the base material, an AlN film on the condition that the temperature of the base material is set to 1100xc2x0 C. or over, and the ratio (V raw material gas/III raw material gas) is set to 800 or over.
In a preferred embodiment of the fabricating method of the present invention, the AlN film is formed under a pressure of 7-17 Torr.
Moreover, in another preferred embodiment of the present invention, the main surface of the base material made of C-faced sapphire single crystal on which the AlN film is formed is nitrided, to form a surface nitride layer at the main surface, and then, the AlN film is formed on the main surface via the surface nitride layer.
Moreover, this invention relates to an acoustic surface wave device, including a substrate composed of a base material made of C-faced sapphire single crystal and an AlN film, formed on the base material, having crystallinity of 90 arcsec or below in FWHM of X-ray rocking curve and a surface flatness of 20 xc3x85 or below, and inter-digital type electrodes.
In the above acoustic surface wave device, it is desired that the main surface of the base material made of the C-faced sapphire single crystal as a constituent portion is nitrided to form a surface nitride layer at the main surface, and the AlN film is formed on the main surface via the surface nitride layer.